A known overheat or fire alarm system comprises a sensor tube in fluid communication with a pneumatic pressure detector, also known as a pressure switch module. The sensor tube commonly comprises a metallic sensor tube containing a metal hydride core, typically titanium hydride, and an inert gas fill, such as helium. Such a system is shown in U.S. Pat. No. 3,122,728 (Lindberg).
Exposure of the sensor tube to a high temperature causes the metal hydride core to evolve hydrogen. The associated pressure rise in the sensor tube causes a normally open pressure switch in the detector to close. This generates a discrete alarm. The pneumatic pressure detector is also configured to generate an averaging overheat alarm due to the pressure rise associated with thermal expansion of the inert gas fill. The discrete and average alarm states may be detected as either a single alarm state using a single pressure switch or separately using at least two pressure switches.
It is also common practice to incorporate an integrity pressure switch that is held closed, in normal temperature conditions, by the pressure exerted by the inert gas fill. A known pneumatic pressure detector having an alarm switch and an integrity switch is shown in U.S. Pat. No. 5,136,278 (Watson et al.). The detector uses an alarm diaphragm and an integrity diaphragm having a common axis.